This invention involves a method and apparatus useful in doing ophthalmological surgery, more specifically, to an improved method of performing laser surgery to correct refractive error, including astigmatism.
Astigmatism is a visual impairment caused by a directional difference in refractive power of the cornea (and or lens) resulting in variations in refraction in the principal meridians of the eye. Prior art surgical techniques to correct the impairment consists of using laser such as a UV laser at 193 nm (excimer) or at 210 nm (diode-pumped fifth harmonic of a Nd:Yag) or an IR laser (erbium) at 2900 nm to ablate the corneal surface to correct the spherical portion of the refractive error as well as the cylindrical portion of the refractive error. The latter has been shaped by using an expanding slit, or a combination of slit and an iris diaphragm mask, to orient the ablation along the desired meridian of the cornea.
One variation of this technique is to use an erodible mask having the appropriate prescriptive power placed over the cornea to guide the surface ablation. One type of such erodible mask consists of a quartz blank with a polymethylmethacrylate (PMMA) lens (Summit Technologies Inc., Waltham, Mass.).
Alternatively, solid state lasers, such as the nano-second YAG and the holmium, are used to treat the corneal stroma in order to modify the surface curvature to correct the spherical and cylindrical refractive error of the eye.
There are notable deficiencies in the prior art techniques for correcting astigmatism with a surgical laser. One deficiency arises from the surgeon's dependence upon current refraction technologies. A second deficiency lies with surgical techniques that treat tissue in a manner resulting in an unwanted coupling effect, as will be further explained below.
Prior art techniques involve treating astigmatism by simply adding or subtracting astigmatism correction as determined by conventional refractive technology. In individual cases where refractive astigmatism is not the same as corneal astigmatism, conventional treatment can lead to an irregular corneal astigmatism due to the production of more than two principal meridians on the corneal surface.
As to the second major drawback to prior art astigmatism correction techniques, prior art interventions suffer from a lack of predictability and from the development of an unwanted coupling effect. Unwanted coupling effect results when the correction of the astigmatic error by surgical ablation creates an unwanted change in the corneal refractive power in the meridian 90.degree. from the treated areas.
Moreover, in cases of compound hyperopic astigmatism and mixed astigmatism, prior art techniques of PRK fail to effectively neutralize the refractive error because of the complexity of the refractive solution. In compound hyperopic astigmatism, there are two focal points, both of which are located posterior to the retina. Prior art attempts to steepen differentially the two principle meridians have met with limited success because of coupling. In the case of mixed astigmatism, one focal point is in front of the retina and the other focal point is behind the retina. In this case, one principle meridian (the myopic focal point) must be flattened while the other principle meridian (the hyperopic focal point) must be steepened. Again, prior art techniques of differentially treating the two meridians have suffered from the adverse effects of coupling.
L'Esperance in U.S. Pat. No. 4,721,379, teaches the use of videokeratoscopic corneal topography data to guide the ablation process. L'Esperance incorrectly taught that corneal topography provides sufficient information to guide the ablation process. (In fact, Roberts has shown that videokeratoscopy provides curvature data but not true topography. True topography gives information about elevation and depression of a surface. True corneal topography alone still can not provide adequate information to guide the ablation process.) Although L'Esperance teaches the use of cornea thickness (pachymetry) as a useful adjunct to corneal topography, it has been determined that PRK (photo-refractive keratectomy) does not depend upon corneal thickness since only superficial cornea is ablated.
The method of the present invention, however, solves, by vector analysis, the direction and magnitude of the underlying lenticular astigmatism. Thus identified, this invention eliminates any corneal astigmatism. The method then corrects the spherical refractive error and the lenticular astigmatic error. The advantage of this approach is that it reduces the risk of irregular astigmatism because only two principal meridians define the corneal refractive surface.
The present invention avoids the unwanted coupling effect by treating the responsible meridian in a graduated fashion and by sparing the center of the visual axis, i.e. the intersection of the line of sight with the cornea. It does this by using a pie-shaped treatment area with a gradual reduction in dioptric correction on either side of the mid-line of the segment. The present invention uses pie-shaped areas of ablation to achieve astigmatic correction. In contrast, Parel (U.S. Pat. No. 5,152,759) taught the use of arcuate shaped laser incision (cuts) to treat astigmatism, much like the arcuate incision made with knives for astigmatism treatment. In U.S. Pat. No. 5,188,636 L'Esperance taught the use of a transition zone at the junction between areas of ablation and nonablation (untreated) areas. In contrast, the present invention features a graded, stepwise ablation throughout the area of astigmatic treatment.
The present invention eliminates all corneal astigmatism by ablating the steepest meridian so that it has the same refractive power as the weaker meridian, thus "sphericiziing" the cornea, and then it treats the resultant spherical hyperopia in compound hyperopia astigmatism and in mixed astigmatism.
In addition to improved astigmatic corrections, the present invention provides a method of improved laser correction of the spherical part of refractive error. Current refraction technologies measure only an average dioptric correction. That is, the normal cornea is aspheric and true refractive error is aspheric and varies across the entrance pupil. Basing laser correction on current refraction technologies results in a spherical correction for an aspheric refractive error. Prior art laser techniques rely on this refractive data alone to calculate the required spherical correction. The technique of the present invention uses a novel apparatus to measure point-to-point refractive data which is then used to guide the laser treatment to a full, customized correction of the individual refractive error.
The present invention teaches that point-to-point refractive error determination over the entrance pupil is a useful way to guide the PRK ablation. Real corneal topography information determined pre-operatively (not during the procedure as taught by L'Esperance) about corneal astigmatism is useful since refractive astigmatism is the sum of corneal astigmatism and lenticular astigmatism. The real corneal topography technology of the present invention can be either a rasterphotogrammetry technique (PAR Vision System, New Hartford, N.Y.), or a laser holography technique (Eye Technology, Inc., St. Paul, Minn.) or any technique of true corneal topography such as the Orbscan (Orbtek, Lake City, Utah). The techniques of the present invention can enhance uncorrected acuity regardless of pupil size (ambient lighting), reduce spherical aberration, and enhance depth of focus. The technique of using rasterphotogrammetry and the like as an adjunct to other types of ophthalmological surgery is described in my co-pending application entitled "Method of Calibrating Lasers for Use in Ophthalmological Surgery," and having Ser. No. 08/055,578, filed May 3, 1993 the disclosure of which is hereby incorporated by reference.